Incorrect installation housings were used. Who bears the resulting costs?

What damage are we talking about here? The client ordered a functioning work that was to be constructed according to recognized rules of construction engineering. The fact that the structural contractor sees it that way as well is shown by the fact that they made corrections, just not professionally. The argument that they don’t earn anything from it also does not apply, since the structural contractor prepared the calculation themselves. You should request the structural contractor to properly construct the ceiling (4 weeks deadline). A retention in the amount of double the removal costs is possible. Just withhold 2,500. The structural contractor must prove to you that their solution was professional and desired. If they still have money outstanding, they will do something.

How would you deal with such a matter now? Would a deduction of 50-60% be okay, or would you say a full 100% deduction!?

So you won’t get far with that. In the final instance, the judge will decide, and it will probably be much less than your numbers. Aside from the stress. The spots are not the core element of a ceiling. This will probably count as an aesthetic defect. If things go well.

What surprises me more is the following

I was not allowed to put in any spots and thus from my side it was: "Take down the ceilings and redo them!" This was not done. The ceilings were just laid on. I had no say, for whatever reason.

You’re the client and you let them do whatever they want...

Now to the solution.

The spots are partly not aligned and not flush with the surface

(?) can you explain that in more detail?

That actually doesn’t matter much, unless they are really completely crooked, but from the photos it looks okay. The covers stay sealed until handover. The diameter of the box is usually much larger than the actual spot, so the proper alignment is only drilled when you move in. In our case, even the painter was already done. Keep the €83 for the spots. Buy new covers for about ten euros and treat yourself to something nice with the remaining €1.50. In the end, everything should still look tip-top.

What damage are we talking about here? The client ordered a functioning work, which had to be executed according to recognized rules of construction technology.

It’s a ceiling and it will probably work. I stick to my opinion. The spots would be an aesthetic defect.